1. Field of the Invention
The present invention relates to industrial mixers for granular products.
2. Description of the Background of the Invention
Mixers for mixing various granular materials together into a relatively homogenous blend come in many different configurations. One traditional mixer configuration is a so-called U-shaped mixer, wherein a blending or mixing apparatus includes one or more agitator assemblies, each including one or more agitators mounted on a horizontal rotatable shaft and carried in an elongate horizontal U-shaped hopper. The U-shaped hopper is sized to closely fit around the mixing apparatus along the length of the shaft so that the agitator(s) engage the entire product in the hopper when the shaft is rotated in order to assure complete blending and/or mixing of the various granular materials. Traditional U-shaped mixers are frequently configured with an agitator that has an auger or ribbons, paddles, or a combination of styles of mixing tools that move the product both axially along the length of the shaft and circumferentially around the shaft in a circular-cylindrical sweep area defined by the rotational periphery of the agitators. The agitator assemblies in such mixers are primarily used to mix the product in the trough without breaking up larger grains or clumps of aggregated product into smaller sizes.
It is also often necessary or desirable to break up larger chunks of the product at the same time the product is being mixed. Therefore, in the past, one or more choppers have been added to mixers in order to help break down and/or reduce the size of agglomerates or clumps or granules that may be created by many conditions, to increase dispersion of particles in the agitator assembly, and/or to provide a desired finish to a granular product being blended. Traditionally, a chopper has been disposed on an inside wall of the trough below the horizontal center of the agitator assembly in order to be buried under the upper surface of the product and ensure that the product is fully engaged by the chopper at all times.
In one example shown in FIG. 1A, a traditional low-speed U-shaped mixer has an agitator 10, such as an auger or a ribbon extending radially from a horizontal shaft, that simply pushes and mixes product 12 in the trough 14 without substantially suspending the product 12 into the air. This mixer does not have the capacity to impart the movement to the product 12 to compare with the performance seen in newer generations of mixer because of the low rotational speed of the agitator 10. Rather, a wedge-shaped region (not shown) of product 12 is formed along the straight sidewall of the trough 14 above the centerline of the agitator 10 and below the top of the agitator 10 that will only have product 12 therein when the volume of product 12 in the trough 14 is sufficient to mechanically push the product 12 upwardly into it. A chopper 16 is disposed on an inside wall of the trough approximately thirty degrees below the horizontal level of the shaft. The chopper has a single shaft extending through the wall of the trough and two sets of radial blades disposed on the shaft and located within the sweep area of the agitators. Therefore, the ribbon or auger must have one or more breaks at the longitudinal location of the chopper 16 in order to provide clearance between the agitator 10 and the chopper 16. These breaks in the ribbon or auger can hurt the efficiency of the mixing/blending action of the mixer. However, the location of the chopper within the sweep area of the agitator buried under the upper surface (shown in phantom) of the product 12 is necessary to ensure that the product is fully engaged by the chopper 16.
Newer mixers often work at higher speeds and can create so-called fluidized zones where product is temporarily suspended in the air. A fluidized zone is a zone where the product is actually temporarily suspended or floating in the air. A fluidized zone is created because the paddles actually lift and throw the product above an at-rest upper surface of the product in such manner that the product is temporarily flying through the air in a fluidized zone when the shaft rotates at higher speeds. For example, FIG. 1B shows a high-speed U-shaped mixer that has two parallel agitator assemblies 18 and a chopper 16. Each agitator assembly 18 includes a plurality of arms that extend radially from a horizontal rotatable shaft in a double U-shaped trough 20 and a paddle disposed at the end of each arm. The agitator assemblies 18 overlap and rotate in opposite directions with the right agitator assembly rotating clockwise (as viewed from either end) and the left agitator assembly rotating counter-clockwise such that the paddles rotate upwardly in a central region where the agitator assemblies overlap. The agitator assemblies 18 have a rotational speed capacity that creates a fluidized zone of product (not shown) in the trough above the area of overlap therebetween, and the product moves generally across the top of the agitator assemblies 18 from the center of the trough 20 toward opposite side walls of the trough 20. Thus, these newer U-shaped mixers are often called fluidizing mixers due to the formation of such fluidized zone or zones within and/or above the region encompassed by the agitator assemblies 18. A detailed example of one type of fluidized mixer, which has two agitators disposed in a double U-shaped trough, is provided in U.S. Pat. No. 4,278,355 to Forberg, which is incorporated by reference herein. In FIG. 1B, the chopper 16 extends into the sweep area of one the agitator assemblies 18 through the side wall of the trough 20 at a location approximately thirty degrees below the shaft of the agitator assembly in order to ensure engagement with the product as the product moves down the sidewalls of the trough. Again, the agitator assembly 18 must be modified to have a break in the sweep area of the paddles in order to avoid hitting the chopper 16, which disrupts a preferred even flow of product through the trough 20.
FIG. 1C shows another type of fluidizing U-shaped mixer that has only a single agitator assembly 22 in a U-shaped trough 14 and a chopper 16 projecting into the sweep area of the agitator assembly 22 below the shaft. In this mixer, the agitator assembly 22 includes a multiple-zone hybrid paddle agitator to mix the product 12 more gently and faster than traditional low-speed mixers and provide flow characteristics of the product 12 that allow the mixer to achieve desired mixing characteristics with the single agitator assembly 22, rather than using the two agitator assemblies 18 like the mixer of FIG. 1B. The agitator assembly 22 has paddles 24 with angular orientations selected to impart desired flow of product that are disposed at the distal ends of arms 26 of different lengths. The different radial placements and angular orientations of the paddles 24 form different radial zones of movement in the trough. For example, the radially inner paddles may move the product 12 in one longitudinal direction along the shaft in an inner zone and the radially outer paddles may move the product 12 in an opposite longitudinal direction along the shaft in an outer zone, thus causing the product 12 to circulate back and forth completely through the trough with only the one agitator assembly 22. The chopper 16 is disposed on an inside wall of the trough approximately thirty degrees below the horizontal level of the shaft in order to be buried under the upper surface of the product and ensure that the product is fully engaged by the chopper 16. Again, the placement of the chopper 16 within the sweep zone of the agitator assembly requires a break in the sweep area of the agitator assembly, which can reduce the efficiency of the circulation and mixing of the product.
A number of difficulties are created by this traditional placement of the chopper below the level of the product at rest. For example, this placement provides a significant amount of resistance to rotating the chopper, which requires a larger motor and more power to start and run the chopper 16. Further, this placement requires a break or opening to be formed in the sweep area along the length of the agitator assembly so that the agitator arms do not hit the chopper during operation, which requires a modification of the agitator arms. This opening leaves an area along the mixer body where product is not mechanically moved directly by the agitator arms, but rather the product is only moved by the motion of the mass of product in the mix. Further, the chopper 16 can disrupt a desired balance flow of the product along the agitator(s) in the trough by re-directing or spraying the product in a different direction because of the spinning of the blades.
The present invention seeks to address one or more difficulties of these prior art designs to provide an improved design for chopper for a mixer.